1. Field of the Invention
The present invention relates to an alignment method for an image reading apparatus for reading image information of an original placed on an original table. In particular, the present invention is suitable for image reading apparatus such as an image scanner, a copying machine, and a facsimile machine, which are capable of high-precision image reading by fully utilizing the optical performance of an imaging optical system having a rotationally asymmetric lens such as an anamorphic lens.
2. Description of the Related Art
Up to now, as apparatus for reading image information of an original placed on an original table, image reading apparatus such as an image scanner and a digital copying machine are known. Of the image reading apparatus, there is known a flatbed type image scanner using an integrated scanning optical unit (carriage-integrated unit) in which an imaging optical system for image reading and a reading unit are housed in a case (carriage).
In this image reading apparatus, the original placed on the original table is illuminated directly or via a reflector with a light beam emitted from an illumination light source. An optical path of the light beam reflected on the original is bent in an inner portion of the carriage through a plurality of reflection mirrors. The image information of the original is imaged by the imaging optical system on a surface of a linear image sensor (hereinafter referred to as “CCD sensor” or “CMOS sensor”) in which reading pixels are arranged in a main scanning direction. The carriage is moved in a sub scanning direction by a sub scanning motor to read the image information of the original. The image information thus read is sent to an external device, for example, a personal computer via an interface.
In recent years, the image reading apparatus having the structure described above is desired to be reduced in size. In order to achieve the reduction in size, it is important to reduce the size of the carriage. In order to reduce the size of the carriage, for example, there are a method of increasing the number of reflection mirrors and a method of performing a plurality of reflections using a single reflection mirror to ensure an optical path length. In addition to this, in recent years, there is proposed an image reading apparatus in which the anamorphic lens having at least one surface with a rotationally asymmetric shape about an optical axis is introduced into the imaging optical system (see Japanese Patent Application Laid-Open No. 2000-171705). In the case of the image reading apparatus, a view angle of the imaging optical system is widened to reduce an object-to-image distance, to thereby shorten the optical path length and reduce the overall size of the apparatus.
When the anamorphic lens is used for the imaging optical system as described above, an image field curvature aberration may be effectively reduced, and hence image information with an excellent contrast may be obtained. However, in the imaging optical system including the anamorphic lens in a rotationally asymmetric shape, the imaging area is also rotationally asymmetric. Particularly in the imaging optical system designed to attain a widened view angle, priority is given to the imaging performance in the main scanning area, which is wide in reading range.
Therefore, the main scanning area has a high resolution over a wide range. In the sub scanning direction, on the other hand, aggravated aberration lowers the resolution, thus narrowing the range of the high resolution. In addition, the carriage-integrated image reading apparatus has many components, and the carriage is in many cases manufactured by resin molding for manufacturing reasons. In general, it is difficult to maintain high positional accuracy for the parts due to a great variation in molding accuracy. For example, if the components such as mirrors are displaced in the sub scanning section, the resolution in the sub scanning direction degrades. In view of the above, there is proposed an alignment method for suppressing the reduction in resolution by aligning the angles of the mirrors in the sub scanning direction and aligning the imaging optical system and the reading unit integrally (see Japanese Patent Application Laid-Open No. 2005-101739).
In the alignment method for an image reading apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-101739, the angles of the mirrors in the sub scanning section are aligned. In this method, when the carriage is downsized to reduce the widths of the mirrors in the sub scanning direction, the sensitivity of the angle alignment tends to be extremely high because the mirrors are reflection members, and hence the alignment is difficult. Further, the imaging optical system is constituted by a rotationally symmetric lens arranged inside a lens barrel and a rotationally asymmetric lens arranged outside the lens barrel. Therefore, if there is a backlash component between the lens barrel and the rotationally asymmetric lens, the rotationally asymmetric lens may be displaced in the sub scanning direction relative to the rotationally symmetric lens inside the lens barrel. At this time, in the method in which the imaging optical system and the reading unit are aligned integrally, it is difficult to perform alignment for contrast degradation due to the displacement in the sub scanning direction, which occurs in the imaging optical system.